Maybe something like this could perform better, based on the following article:

http://www.sqlservercentral.com/articles/T-SQL/91724/

CREATE FUNCTION [dbo].[DateFormattedByMask]

(

@InputDate DATETIME,

@Mask NVARCHAR(80)

)

RETURNS TABLE

AS

RETURN

(

SELECT Mask = REPLACE(

REPLACE(

REPLACE(

REPLACE(

REPLACE(

REPLACE(

REPLACE(

REPLACE(

REPLACE(

REPLACE(

REPLACE(@Mask,'hh', RIGHT('0'+DATENAME(hh,@InputDate),2))

,'mi', RIGHT('0'+DATENAME(mi,@InputDate),2))

,'ss', RIGHT('0'+DATENAME(ss,@InputDate),2))

,'YYYY',DATENAME(YEAR,@InputDate))

,'YY',RIGHT(DATENAME(YEAR,@InputDate),2))

,'MMMM', DATENAME(MONTH,@InputDate))

,'MMM', LEFT(DATENAME(MONTH,@InputDate),3))

,'MM', RIGHT('0'+CAST(MONTH(@InputDate) AS NVARCHAR(2)),2))

,'DDDD', DATENAME(dw,@InputDate))

,'DDD', LEFT(DATENAME(dw,@InputDate),3))

,'DD', RIGHT('0'+DATENAME(DAY,@InputDate),2))

)

GO

EDIT: format and minute code issues.